Features

Polystyrene Blown Film Starts to Get Some Respect

By
Jan H. Schut

From: Plastics Technology
Issue: November 2000

It has kept a low profile for the last 40 years, but more blown-film processors are starting to take a look at PS for breathable packaging and cellophane replacements. A couple of dozen processors worldwide have kept this technology quietly to themselves—but not for much longer.

Polystyrene is rarely thought of as a blown-film resin. Machinery suppliers don’t sell equipment for it. Resin producers don’t offer blown-film grades of it. (There are very few grades of PS even for cast film.) But in fact, polystyrene not only can be blown into high-quality film, it has been for decades. Plain general-purpose PS has been blown commercially since the 1960s on modified but fairly standard equipment. You just haven’t heard much about it, because those who were doing it kept quiet. That could be because this low-cost resin has found its way into high-value applications that are quite profitable for the few players that have pursued them.

Polystyrene has found a niche in monolayer and multi-layer blown films because it is stiff, clear, and printable. It also has good deadfold properties, and its gas transmission or “breathability” is an increasingly attractive property for packaging fresh foods.

The advantages of blown PS film over cast film are that it uses much less expensive machinery, adds more orientation, and can make much thinner gauges. For example, Dow Plastics’ Fabricated Products Div. in Hebron, Ohio, blows its Trycite PS films 1 to 10 mils thick. Avpex International in New Castle, Ont., blows PS films of 1 to 4 mils and casts PS films over 4 mils thick.

Blown PS film also has the advantage that it can be made with either matte or glossy surfaces, whereas cast film can only be glossy. Like cast film, blown PS achieves high clarity, unless it is blended with enough styrene-butadiene copolymer to become hazy.

There are also disadvantages to PS as a blown-film resin. One is that it’s difficult to process, especially as a monolayer material. The hurdles are lower when PS layers are sandwiched between polyolefins. There’s little technical literature available on how to do it—just one or two TAPPI or SPI ANTEC papers and a couple of articles published in Europe, all dating from the 1980s. The equipment currently in use for the process is all proprietary and assembled by processors in-house. Machinery suppliers have built some commercial lines, or parts of them, but always to proprietary customer specs and under secrecy agreements. Exceptions are multi-layer blown-film lines designed to combine layers of polyolefins and modified PS. The latter are, in fact, fairly conventional systems, since the polyolefin layers generally support the styrenic ones.

Blown PS film acquires high orientation—higher even than cast OPS film. That may be an advantage in some uses, but can also be a drawback for thermoformers because of high shrinkage.

Impact strength of blown PS also typically ranges from poor to “not quite so poor,” notes Ken Cherry, business manager for elastomers at Atofina Petrochemicals (formerly Fina Oil & Chemical). However, virtually all PS blown film is modified to some degree with either HIPS or a rubbery styrene-butadiene copolymer (SBC) such as K-Resin from Chevron/Phillips or Atofina’s Finaclear. With addition of 5 to 30% SBC, PS blown film moves up to the “not-quite-so-poor” toughness category. Adding HIPS is less effective.

Durometer of blown PS film can range from rigid to elastomeric, depending on the amount and type of SBC mixed in. The SBCs used to modify blown films are generally high-styrene types, roughly 75% styrene/25% butadiene. They are not designed specifically for film.

Interest is picking up

There’s no stampede of newcomers rushing into blown PS film. But interest in it is definitely picking up. Several machinery suppliers say that in the past year or two they have been asked for the first time to run blown-film trials with PS. Some cast-film processors are thinking about getting into blown PS, especially companies that have acquired expertise in this area through mergers.

Plastic Suppliers in Columbus, Ohio, for example, makes cast and tentered OPS film and has distributed Dow’s blown PS film for years. Now Plastic Suppliers is thinking of making blown PS film on its own. The technology would come from Sidaplax NV, a Belgian PS film processor that Plastic Suppliers bought in the mid-’80s. Sidaplax primarily makes cast film but also makes blown PS film for labels that go inside meat packages. Plastic Suppliers is considering using that technology at a new plant in Columbus.

Meanwhile, Avpex’s parent, Kama Corp. in Hazleton, Pa., is considering transferring the Avpex blown PS film technology it acquired in North America to its new U.K plant. Kama only makes cast film in Europe, though it distributes Avpex blown PS film abroad.

Modified PS finds niche markets in monolayer and multi-layer blown films. The biggest use, which emerged in the past seven to eight years, is in breathable films for fresh produce. These films are known as modified-atmosphere packaging or MAP. PS is a hit because its crinkly feel conveys the idea of freshness to the consumer. MAP applications have nearly doubled every year for the past five years or so, says Bill Wright, president of Barrier Films Corp., the world’s largest producer of MAP film, headquartered in Sparks, Nev.

The level of breathability required of a food package determines the amount of SBC modifier used because it has higher gas-transmission rates than pure PS. Fruit that ripens in the package might use 20-30% of SBC to allow freer passage of oxygen in and CO2 out. Produce that respires less might use 5-10% SBC, just for impact improvement.

Barrier Films extrudes more than 1 million lb/yr of SBC-modified PS into two layers of a seven-layer MAP film, called Aluphane. PS/SBC composes about 30% of the film by weight. First commercialized in 1992, the film is “very tough and very effective at emulating oriented PP,” Wright says. Barrier Films uses K-Resin grade DK-11 and other SBCs. Colors go only in the middle olefinic layer with EVA barrier layers on either side to isolate the colors and prevent them from bleeding out of the film onto oily foods.

Probably a dozen processors are blowing MAP films that include SBC-modified PS. They include Amko Plastics Inc. in Cincinnati and the former Cyprus Bags in Rochester, N.Y., now a unit of Cryovac/Sealed Air.

Blown PS film is finding other uses in food packaging. Permapack AG in Rorschach, Switzerland, for example, developed two- and three-layer PS films for fresh-meat packaging three years ago. Permapack makes rigid films to thermoform into meat trays. One consists of EVA (3 microns) and highly SBC-modified PS (10 microns). A thicker (16-micron) Permapack film that is formed into trays for whole chickens and meat on the bone has 3-micron layers of EVA on either side of a SBC/PS core layer.

Replacing cellophane

Other markets for blown PS film are the same as for cast PS—envelope windows, hard-candy wrappers, labels, and other cellophane replacements where PS’s deadfold, crinkle, and other paper-like qualities are a plus. Very thin PS film (4 mils), for example, is laminated to paper for greeting cards and can be converted on conventional paper equipment. Candy wrap is showing the strongest growth among these uses.

Other specialty cellophane-replacement markets are clear tinted wrap for food baskets and flowers and glossy printable films for laminating onto thermoformable sheet for point-of-purchase displays. Recent applications in Europe include clear pages for stamp albums, sleeves for document protection, and sheets for packages of meat and fish.

Dow was probably the first processor to blow PS films for cellophane replacement in the 1960s on slightly modified monolayer film equipment with an “up” bubble. Dow has kept its Trycite film process in Hebron, Ohio, quietly under wraps.

Dow today has several blown film lines running monolayer and multi-layer PS. The older ones are very small, the later ones are large. The biggest has up to a 160-in. layflat with a 6:1 blow up ratio. Dow produces several grades of general-purpose PS exclusively for its film operations. Most of these films are modified with HIPS.

In the ’80s, Dow built a second blown-PS plant in Drusenheim, France, and Sidaplax started blowing PS film in Gent-Bruge, Belgium. Europlast Srl in Bari, Italy, started blowing monolayer PS for envelope windows and labels in 1995. It’s four lines were built in-house.

A low level of foaming is used in blown PS film run by Owens-Illinois at its plant in Bardsville, Ky. That film is produced with a horizontal bubble and is used for soft-drink labels. The foam helps keep the bottle cold.

Processing challenges

Most machinery suppliers have limited experience with blown PS film. Addex Inc. in Hingham, Mass., has supplied a whole line for blown PS, but it was built to proprietary customer specs. Windmoeller & Hoelscher GmbH in Germany (with U.S. offices in Lincoln, R.I.) has run customer trials in the past year and a half for the first time. It was surprised to find that PS ran on unmodified blown-film equipment. Battenfeld Gloucester has also received inquiries about blown PS film in the past year but has not yet built equipment for it or run trials.

“Everything about blown PS is more difficult,” notes the president of a machinery company that has supplied PS blown-film die heads and air rings under secrecy agreements but has never built a whole PS line. Resin companies and processors who have tried blowing PS film warn that it has to be run much more slowly and carefully than polyolefin films. “Every film extruder wants to run as fast as they can,” says Atofina’s Cherry. “But with blown PS, heat stability is important. You can’t overheat it or you get bubble flutter and uneven gauge.”

The processing-temperature window is very narrow, say knowledgeable sources. Also, a PS bubble cools far more rapidly than a polyolefin one, so the bubbles are only 10-12 ft high and have a relatively high stalk. The blow-up ratio is 6:1 in both MD and TD directions, for an overall stretch ratio of 36:1.

PS is extruded with screen packs and filters like PE film, but PS screens are finer because the threat of gels is greater. Gels may be unsightly in PE film, but they will collapse a PS bubble.

PS film also scratches easily and edges have a tendency to crack. So all aspects of handling and winding take greater care, processors say. Rollers must be free-running and rubber coated, like rollers for tacky film. In most cases, processors trim off the edges of the layflat at the fold.

Know your modifiers

Since styrenic modifiers are so important to the properties of PS film, anyone contemplating this field must become acquainted with what’s available. Although HIPS is of limited help in boosting properties of PS film, some newer grades have more to offer. For example, Dow’s Styron A-Tech (Advanced Technology) HIPS, introduced two years ago, incrementally improves gloss and impact properties by combining two different butadiene rubbers to achieve bimodality.

A sudden shortage of SBC arose after an explosion last March at Chevron/ Phillips’ 300-million-lb/yr K-Resin plant in Pasadena, Texas. Chevron/Phillips continues to supply K-Resin from Korea and hopes to restart its Texas plant in the first quarter of next year.

In the meantime, the K-Resin crunch has had the beneficial effect of stimulating new products and new capacity from other producers. Kraton Polymers has come out with an equivalent SBC grade (D 1401P), its first Kraton product for packaging. Calsak Corp. introduced K-Resin alternatives from Denka of Japan. One aimed particularly at extrusion is Clearen 730L.

Atofina plans to double production of its Finaclear SBC at Antwerp, Belgium, next year to 90 million lb/yr. And BASF will expand global production of its Styrolux SBC by 150 million lb/yr, through a capacity addition in Germany and new plants in Mexico and Belgium. The new production will include a new grade, Styrolux BX 6230, which BASF is sampling in the U.S. now. It reportedly has enhanced properties that allow less modifier to achieve the same properties.

Dow’s developmental “Index” ethylene-styrene interpolymer (ESI) is a new alternative to either SBC or HIPS for improving PS’s impact properties. It gives PS impact properties that reportedly beat those of PP. Earlier this year, Dow Fabricated Products commercialized the first blown film to use up to 50% ESI in GPPS. This 1.5-mil blown film is heat laminated onto 1-in. Styrofoam insulation boards. The film is strong enough to allow the foam to be walked on by construction workers. The film also resists damage when nails or staples are pulled out of the boards. “You would never believe it was PS,” says Bill LaFollette, product development manager for the film, which will replace PP film on all Dow’s Styrofoam Duramate insulation boards. It’s tougher than PP, doesn’t require adhesive, and facilitates recycling of scrap board.

MAP films for fresh produce packaging are a hot market for PS blown film, and Barrier Films is the biggest of around a dozen U.S. producers.